Enterotoxigenic Escherichia coli (ETEC), one of several pathotypes of diarrheagenic E. coli, causes a secretory-type diarrhea ranging from mild to cholera-like purging. ETEC poses an important medical concern to persons living in and travelers visiting many developing countries. ETEC is a principal cause of diarrhea in young children in resource-limited countries and also travelers to these areas (Black, 1990; Huilan, et al, 1991). Among infants and young children, the organism is estimated to cause 210 million cases of diarrhea and 380,000 deaths annually (Qadri, et al, 2005).
ETEC produce disease by adherence to small intestinal epithelial cells and expression of a heat-labile (LTI) and/or heat-stable (ST) enterotoxin (Nataro, et al, 1990). ETEC typically attach to host cells via filamentous bacterial surface structures known as colonization factors (CFs). More than 20 different CFs have been described, a minority of which have been unequivocally incriminated in pathogenesis (Gaastra and Svennerholm, 1996).
Firm evidence for a pathogenic role exists for colonization factor antigen I (CFA/I), the first human-specific ETEC CF to be described. CFA/I is the archetype of a family of eight ETEC fimbriae that share genetic and biochemical features (Evans, et al, 1975; Gaastra and Svennerholm, 1996; Grewal, et al, 1997; Khalil, et al, 2000). This family includes coli surface antigen 1 (CS1), CS2, CS4, CS14, CS17, CS19 and putative colonization factor O71 (PCFO71). The complete DNA sequences of the gene clusters encoding all eight members of this fimbrial family have been published (Froehlich, et al, 1994; Froehlich, 1995; Jordi, et al, 1992; Perez-Casal, et al, 1990; Scott, et al, 1992). The four-gene bioassembly operons of CFA/I and related fimbriae are similarly organized, encoding (in order) a periplasmic chaperone, major fimbrial subunit, outer membrane usher protein, and minor fimbrial subunit. CFA/I assembly takes place through the alternate chaperone pathway, distinct from the classic chaperone-usher pathway of type I fimbrial formation and that of other filamentous structures such as type IV pili (Ramer, et al, 2002; Soto and Hultgren, 1999). Based on the primary sequence of the major fimbrial subunit, CFA/I and related fimbriae have been grouped as Class 5 fimbriae (Low, et al, 1996).
Studies of CS1 have yielded details on the composition and functional features of Class 5 fimbriae (Sakellaris and Scott, 1998). The CS1 fimbrial stalk consists of repeating CooA major subunits. The CooD minor subunit is allegedly localized to the fimbrial-tip, comprising an extremely small proportion of the fimbrial mass, and is required for initiation of fimbrial formation (Sakellaris, et al, 1999). Contrary to earlier evidence suggesting that the major subunit mediates binding (Buhler, et al, 1991), recent findings, therefore, have implicated the minor subunit as responsible for fimbria-mediated adhesion and identified specific amino acid residues required for in vitro adhesion of CS1 and CFA/I fimbriae (Sakellaris, et al, 1999). The major subunits are responsible for serological distinctiveness of each fimbrae with the minor subunits (Gaastra, et al, 2002).
Comparative evolutionary analyses of Class 5 major and minor subunits demonstrate that greater structural conservation exists among the minor subunits as compared to the major subunits. This is consistent with ability of anti-minor subunit but not anti-major subunit or fimbrial antibodies to inhibit mannose-resistant hemagglutination (MRHA) of ETEC that express heterologous, subclass-related fimbriae (Anantha, et al, 2004).
Prior research efforts in uropathogenic E. coli strains, containing Type 1 and P fimbriae, have been used as models to elucidate the mechanisms of assembly of pili on these strains of bacteria. These studies showed that assembly in uropathogenic E. coli is effected via a chaperone-usher pathway (Kuehn, et al, 1992; Sauer, et al, 1999; Choudhury, et al, 1999). An outcome of this work has been development of the principle of donor strand complementation, a process in which fimbrial subunits non-covalently interlock with adjoining subunits by iterative inter-subunit sharing of a critical, missing β-strand (Sauer, et al 1999; Choudhury, et al 1999; Barnhart, et al, 2000). Evidence has implicated this same mechanism in the folding and quaternary conformational integrity of Haemophilus influenzae hemagglutinating pili (Krasan, et al, 2000), and Yersinia pestis capsular protein, a non-fimbrial protein polymer (Zavialov, et al, 2002). Both of these structures are distant Class I relatives of Type 1 and P fimbriae that are assembled by the classical chaperone-usher pathway.
Despite the efforts in uropathogenic E. coli, the identity of the adhesion moieties and the mechanism of fimbriae assembly in ETEC have been unclear. That the fimbrial assembly and structural components of these distinct pathways share no sequence similarity suggests that they have arisen through convergent evolutionary paths. Nevertheless, computational analyses of the CFA/I structural subunits suggest the possibility that donor strand complementation may also govern chaperone-subunit and subunit-subunit interaction.
The eight ETEC Class 5 fimbriae clustered into three subclasses of three (CFA/I, CS4, and CS14), four (CS1, PCFO71, CS17 and CS19), and one (CS2) member(s) (referred to as subclasses 5a, 5b, and 5c, respectively) (Anantha, et al, 2004). Previous reports demonstrated that ETEC bearing CFA/I, CS2, CS4, CS14 and CS19 manifest adherence to cultured Caco-2 cells (Grewal, et al, 1997; Viboud, et al, 1996). However, conflicting data have been published regarding which of the component subunits of CFA/I and CS1 mediate adherence (Buhler, et al, 1991; Sakellaris, et al, 1999).
This question of which fimbrial components is responsible for mediating adherence was approached by assessing the adherence-inhibition activity of antibodies to intact CFA/I fimbriae, CfaB (major subunit), and to non-overlapping amino-terminal (residues 23-211) and carboxy-terminal (residues 212-360) halves of CfaE (minor subunit) in two different in vitro adherence models (Anantha, et al, 2004). It was demonstrated that the most important domain for CFA/I adherence resides in the amino-terminal half of the adhesin CfaE.
The studies briefly described above provide evidence that the minor subunits of CFA/I, as well as the homologous subunits of other Class 5 fimbriae, are the receptor binding moiety (Sakellaris, et al, 1999; Anantha, et al, 2004). Consistent with these observations, because of the low levels of sequence divergence of the minor subunits observed within fimbrial subclasses 5a and 5b (Sakellaris, et al, 1999), the evolutionary relationships correlated with cross-reactivity of antibodies against the amino-terminal half of minor subunits representing each of these two subclasses (Anantha, et al, 2004).
Similar, but distinct from Class 5 fimbriae, coli surface antigen (CS3) represents the common adhesive fibrillae of the ETEC colonization factor antigen II (CFA/II) complex. ETEC expressing these antigens are prevalent in many parts of the world. CS3 is composed of two subunits, CstH and CstG. Furthermore, anti-sera against CstH, but not CstG, exhibited hemagglutination inhibition, suggesting that the CstH was the CS3 adhesin.
The CS3 fibrillar assembly has been classified as a member of the classical chaperone-usher (CU) pathway based on the genetic relatedness of the CS3 periplasmic chaperone to the PapD superfamily (Hung, et al, 1999). Interestingly, it falls into the FGL (F1-G1 long) subfamily, referring to a characteristic structural feature of the chaperone, which mediates assembly of thin fibrillar or afimbrial adhesive organelles (Soto and Hultgren, 1996). Alignment of the N-terminal amino acid span of CstH with Yersinia pestis F1 capsule subunit reveals a common motif of alternating hydrophobic residues through amino acid 16 (with reference to the mature CstH polypeptide). This span of the F1 capsular subunit (Caf1) functions as the donor strand, interacting with the Caf1M chaperone and neighboring F1 protein subunits during capsular assembly and subunit articulation (Zavialov, et al, 2003). Therefore, it is logically reasoned that CstH may function in a similar manner.
Cholera toxin (CT) and E. coli enterotoxins (LTI and LTII) are members of the heat-labile enterotoxin family (Hirst, 1999; Holmes, 1997; Jobling and Holmes, 2005). They act on enterocytes of the small intestine and cause secretory diarrhea. Each toxin consists of a single A polypeptide and five identical B polypeptides all attached by noncovalent interactions. The known variants of CT and LTI belong to serogroup I and variants of LTII to serogroup II.
Structures of CT, LTI and LTIIb show that all have closely related folding patterns, despite differences in amino acid sequences between the B polypeptides of toxins in serogroups I and II (Domenighini, et al, 1995). The five identical B polypeptides form a doughnut-shaped module. The A polypeptide has an A1 domain located next to the upper face of the B subunit and an A2 domain that penetrates the central pore of the B pentamer. A1 and A2 are joined by a short surface-exposed loop. Proteolytic cleavage within that loop generates nicked holotoxin, with fragments A1 and A2 remaining linked by a disulfide bond. Five identical binding sites on the lower face of the B pentamer interact with specific receptors on target cells. The receptor-binding specificities among the enterotoxins differ greatly. CT and LTI bind tightly to ganglioside GM1. LTI, but not CT, binds to asialoganglioside GM1 and certain glycoproteins, and LTIIa and LTIIb bind best to gangliosides GD1b and GD1a, respectively.
The activity of enterotoxins on cells, such as epithelial cells upon colonization by ETEC, is mediated by an intricate sequence of events (Hirst, 1999; Holmes, 1997; Jobling and Holmes, 2005; Spangler, 1992). Upon colonization, ETEC heat-stable (ST) and/or heat-labile (LTI) enterotoxin act upon epithelial cells. In addition to LTI, ETEC heat-stable enterotoxin (ST) is a nonimmunogenic peptide analog of the intestinal peptide guanylin that activates intestinal membrane-bound guanylate cyclase (Schulz, et al, 1997).
Seroepidemiologic studies of young children has shown an inverse correlation between serum anti-CFA/I IgG antibody levels and a risk of disease with CFA/1-ETEC (Rao, et al, 2005). However, studies have failed to demonstrate that anti-LTI antibodies are protective. Evidence exists that administration of the B subunit of CT (CT-B) confers significant protection against ETEC caused diarrhea, which express the antigenically similar LTI enterotoxin (Clemens, et al, 1988; Peltola, et al, 1991). Furthermore, animal challenge studies have suggested that anti-fimbrial and anti-LTI antibodies act synergistically to protect against ETEC challenge (Ahren and Svennerholm, 1982).
Because of the promising immune responses to CFA/I and other coli surface antigens with nontoxic forms of LTI or CT, these antigens have been the focus of mucosal vaccine formulations against ETEC (Holmgren and Czerkinsky, 2005). An oral, killed whole-cell ETEC vaccine co-administered with CT-B has been extensively tested (Savarino, et al, 1999; Svennerholm, et al, 1997). Although the vaccine was found to be safe, it was not efficacious in infants (Savarino, et al, 2003). Furthermore, live attenuated ETEC vaccines have not proven effective partly due to the lack of achieving a proper balance between attenuation and immunogenicity (Altboum, et al, 2003; Barry, et al, 2003; Levine, et al, 1984; Turner, et al, 2001). Therefore, the importance of identification of the fimbrial component that might more effectively induce anti-adhesive immunity has become ever more acute. In ETEC, this moiety has been shown to be the minor fimbrial subunits, such as CfaE. Therefore, an aspect of this invention is the construction and use of conformationally stable ETEC fimbrial adhesins or adhesin domains in conjunction with components of bacterial toxoids, such as CT or LT, to induce immunity against diarreheagenic ETEC.